Antidepressant drugs are reported to decrease metabolic rate and alter body mass of depressed patients. Increased body mass may be partly related to the decrease in metabolic rate. The change in energy budgeting that follows chronic drug treatment may be related to the behavior activating effects of antidepressant drugs. Previously we have found that chronic inhibition of type A monoamine oxidase (MAO) with the antidepressant drug clorgyline a) alters body mass, b) decreases body lipid content, c) decreases oxygen and food consumption, and d) decreases the level of peritoneal and brain temperature in Syrian hamsters. Studies conducted in the past year have indicated that clorgyline treatment alters adrenal, kidney and testicular masses suggesting possible endocrine effects of clorgyline-treatment. Preliminary data collected in hamsters indicates that clorgyline treatment activates brown adipose tissue thermogenesis primarily during the active phase of the daily activity-rest cycle. This increased thermogenesis is possibly related to decreased body lipid content in chronically treated animals. Based on the observations that clorgyline decreases hypothalamic temperature and oxygen consumption, we have hypothesized that MAO inhibition decreases the thermoregulatory set-point of the hypothalamus. To further test this hypothesis, we have initiated a collaborative study with Dr. Christopher Gordon to evaluate the temperature preference of clorgyline-treated hamsters living on a temperature gradient. In sum, these studies are providing valuable information regarding alterations in daily energy budgeting following antidepressant drug treatment, and promise to be fruitful areas of investigation.